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Delabole Slate. Superior handcrafted Cornish slate products direct from the world renowned Delabole Slate Quarry in Cornwall.<br>
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Technical Information

Mineralogical Composition

The individual mineral crystals in Delabole Slate are not visible to the naked eye. Under the high-power microscope the slate is seen to be a dense, uniform, microcrystalline metamorphic rock, the mineral grains of which have a flaky habit and a thickness of the order of 5 microns (see fig 1). The well developed cleavage of the slate is due to the flaky habit of the mineral crystals formed under intense pressure and matted together in a felt-like mat of sericitic mica, chlorite, quartz and minor proportion minerals all compacted together in parallel orientation. The major proportion minerals and some of the minor ones are listed in Table 1 which also shows the chemical analysis compounds which enter into their composition.


Particle Shape

Because of the flaky habit of the component minerals - chiefly sericitic mica, the particles of slate powder & granules are also flaky, not sharp or angular; they have a thickness of one quarter to one tenth of their breadth. In many applications this characteristic has been found to have reinforcing properties. The particle shape gives a high specific surface and consequently good covering capacity.


Chemical Analysis

The chemical analysis of Delabole Slate powder & granules is given in Table 1. Much of the silica in Delabole Slate fillers is locked up in very stable chemical compounds, ie, silicates of alumina, iron, magnesium, potash, etc. These compounds appear as minerals, such as the abundant sericitic mica and chlorite and minor proportion minerals which give Delabole Slate fillers special, desirable physical properties. Table 1 shows the distribution of the chemical analysis compounds in the component slate minerals. It is these abundant, stable, low abrasive, low moisture absorption, smooth, platy minerals, which make slate powder & granules an ideal mineral filler or carrier for many purposes.

Water soluble salts

Sodium Oxide = 0.006%
Potassium Oxide = 0.037%
Calcium = 0.002%
Magnesium = 0.005%

Total insolubles and solubles (When Ground)

Soluble (Hydrochloric Acid) = 13.1%
Insoluble (Hydrochloric Acid) = 86.9%



Moisture Content & Absorption

Moisture absorption in accordance with BS: 812: Part 2: 1975, where appropriate. The moisture absorption was found to increase with fineness but with a very close range between the finest and the coarsest grade; the moisture absorption did not exceed 1.7%. The material was sealed in a container under maximum humidity conditions to achieve this result.


Oil Absorption

In accordance with BS: 3483:

Part B7: 1982

The oil absorption value will vary according to fineness of the filler.

The range is as follows:

99-350 Grade = 44.1 ml per 100 gms
70-200 Grade = 33.9 ml per 100 gms


PH Value

In accordance with BS: 1975: 1976

99-350 Grade 9.2
70-200 grade 9.1


Fig 1. and Fig 2. Photomicrograph of Delabole Slate



Table 1. Chemical Analysis and Mineralogical Relationships of Delabole Slate Powder and Granules

Please Click on the Table above to enlarge.

Specific Surface

In accordance with BS: 4550 : Part 3 : Section 3.3.

The specific surface will vary according to fineness and particle shape. As ‘Delafila’ has a flaky particle shape the Specific surface is relatively high.

The ranges are as follows:

99 - 350 grade = 10,975 cm2 /gram
70 - 200 grade = 7,210 cm2 /gram


Relative Density

In accordance with BS: 812 : Part 2 : 1975

The relative density of ‘Delafila’ is 2.8


Bulk Density

In accordance with BS: 812 : Part 2 : 1975

Bulk density will vary according to particle shape, size etc.

The ranges are as follows:

    F20 Grade 70 - 200 Grade
In toluene   0.23 g/ml 0.56 g/ml
Loose state   444 kg/m3 833 kg/m3
Compacted state   1273 kg/m3 1595 kg/m3


MOHS Hardness

The hardness of the powder is a function of the minerals present and will vary from 2 - 7.



The abrasive properties of mineral fillers depend on two factors:

  1. Mineral composition
  2. Fineness grading

A fine powder is less abrasive than a coarse powder of the same mineralogical composition. Delafila 99-350 grade is about half as abrasive as Delafila 70-200 BSS. A quartz powder can be 7-10 times as abrasive as 99-300 BSS, and some commercial talcs are more abrasive than Delafila.


Electrical Conductivity

Carried out on 1% w/v in de-ionised water.

Conductivity will vary according to particle size for a suspension.

The ranges are as follows:

99 - 350 grade = 7.87 (uS/cm)
70 - 200 grade = 15.50 (uS/cm)


Environment Health & Safety

Slate is a naturally occurring material that is inert and non-polluting in its solid state. Pollution prospects arise from powdered slate, which should be kept slurrified or confined.


Waste Disposal

Unwanted material should be disposed of in accordance with the “Duty of Care – Control of Waste Regulations”, and any local Government requirements (Note: pollution prospects of powders as above).


Hazards to Health

Hazards are generally associated with poor handling and dust. Dust can be harmful to health.


Precautions - Handling:

  • operating lifting capabilities must exceed 25kg
  • use mechanical devices where possible
  • avoid the generation of dust
  • use wet, if process allows
  • if dust occurs, use a suitable dust mask



A Powder & Granules brochure is available which includes more detailed information. Please select "BROCHURES" from the menu at the top or click here.


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